We are modifying the immunological properties of various enzymes by covalent attachment of polyethylene glycol to produce enzymes with extended circulating lives in the blood following intravenous injection, and with little or no observable immunogenicity. Enzymes being so modified include uricase, catalase, phenylalanine ammonia-lyase, trypsin, asparaginase, arginase, carboxy peptidases. In addition, antibodies are being modified in an effort to reduce their antigenicity in recipients while retaining their antibody function. Allergens are being modified to bias their production of antibodies in favor of IgG-type.